FIG. 4 illustrates an example of basic connection of an apparatus for controlling a thrust force of an air cylinder by an air servo valve. In the figure, with respect to reference numbers, 1 denotes an air cylinder, 2 denotes a three-position air servo valve connected to a head-side pressure chamber 1a of the air cylinder 1, 3 denotes a pressure air source connected to the air servo valve 2 and a rod-side pressure chamber 1b through a regulator 4, 5 denotes a controller controlling the air servo valve 2 by a PID adjuster 5a (see FIG. 5), 6 denotes a pressure sensor detecting an air pressure in the head-side pressure chamber 1a and feeding back a signal of the detected pressure to the controller 5, and 7 denotes a position sensor detecting the position of a piston 1c of the air cylinder 1.
In the apparatus, when the air servo valve 2 is switched to a first position, shown on the left of the figure, by the controller 5, and a pressure air is fed to the head-side pressure chamber 1a of the air cylinder 1, the piston 1c and a rod 1d of the air cylinder 1 move forward to the right in the figure. On this occasion, a pressure in the head-side pressure chamber 1a is detected by the pressure sensor 6, the position of the piston 1c is detected by the position sensor 7, and respective detection signals are fed back to the controller 5. Then, by applying a necessary gain (amplification) on a difference between a pressure instruction value and the detection value of the pressure in the PID adjuster 5a of the controller 5, and controlling the air servo valve 2, thrust control in accordance with the position of the piston 1c is performed. On this occasion, the air servo valve 2 is opened at the degree of opening in accordance a control signal having the gain applied thereon, and the pressure in the pressure chamber 1a of the cylinder 1 is controlled with a rate of airflow according to the degree of opening.
FIG. 5 is a block diagram of the apparatus, for controlling a thrust force of the air cylinder 1 by controlling the pressure in the pressure chamber 1a. With respect to reference characters in the figure, “Pi” is an instruction value, “Kp” is a proportional gain of the PID adjuster 5a, “G(S)” is a transfer function of the air servo valve 2, “V” is a volume of the pressure chamber, “1/VS” is a transfer function of the air cylinder 1, “a” is a constant, “T” is a time constant, “s” is Laplace operator, “Q” is a manipulated variable, “Po” is a controlled variable, and “Kc” is a feedback gain. The block diagram will be described in detail later, in association with the description of the present invention.